1. Field of the Invention
This invention relates to the storage of data in nonvolatile memory within hermetically sealed devices, specifically implantable medical devices.
2. Description of the Prior Art
In certain applications of electrical devices, it is necessary to isolate the electrical circuits or components of the devices from the environment which makes it difficult to access the circuits or components to make any adjustments in component values or data stored therein after enclosure is completed.
Moreover, the completion of the manufacturing process may affect the components within the enclosure. In the field of implantable medical devices, such as human tissue stimulators or drug dispensers, the electronic components, power source and other electromechanical components are typically sealed within a housing or enclosure to protect the enclosed components from body fluids. Thereafter, access is limited, typically, to signal transmission through dedicated feedthroughs for specific functions related to the delivery of a therapy to the patient or detection of specific body conditions or signals. In order to change the operating parameters, or modes of the medical device, or in order to retrieve data from memory or sensors coupled to the device, it has become customary to provide a communication link by uplink and downlink RF telemetry. Thus, after final assembly, and subsequently after implant, communication is typically effected through application of a radio frequency carrier field to the device by an external programmer/transceiver. Examples of such communication are described in Medtronic U.S. Pat. No. 4,250,884 and the article entitled "Microcomputer-Controlled Devices For Human Implantation" in Johns Hopkins API Technical Digest, Vol. 4, No. 2, 1983, pp. 96-103 by R. E. Fischell. In addition, the Ellinwood U.S. Pat. No. 4,146,029 discloses both RF telemetry and direct needle access communication to and from an implantable pacemaker/drug dispenser for programming mode and parameter values of operation of the device and retrieval of any stored data.
In the aforementioned prior art medical devices, the contents of volatile memory are programmed in or read out by downlink and uplink telemetry, respectively, or direct access (Ellinwood). The prior art medical devices are implemented in either discrete digital logic and storage register or in microprocessor based system architecture including nonvolatile ROM and volatile RAM memory, as shown for example in FIG. 24 of the Ellinwood patent and page 98 of the Fischell article.
In the development of such microprocessor based implantable medical devices, it is customary to construct prototype breadboards to optimize the functions, modes and parameters of intended operation of the device and to program and debug the software employing, at that stage of development, ultraviolet light erasable PROMS or EEPROMS to facilitate design changes. Once the design is frozen, the circuitry is miniaturized and optimized for manufacturability, reliability and longevity employing custom integrated circuitry, and permanently programmed ROM and volatile RAM memory. In the completed devices, only the contents of the RAM may be subsequently altered in the fashion described hereinbefore.
In addition such medical devices include analog circuitry with discrete resistors and capacitors in hybrid circuit packages wherein the values of the resistors and capacitors are mechanically "trimmed" to meet the operational specificities of the circuit. In this procedure the output of the circuit is made to conform to a specified value for a specified input.
The increased level of sophistication of implanted electronic medical devices manifests itself in increased capacity for data storage and retrieval as well as customization of the device functions and parameters to the patient condition.
In regard to cardiac pacemakers, early pacemakers provided a fixed rate stimulation pulse generator that could be reset on demand by sensed atrial and/or ventricular depolarizations. Modern pacemakers include complex stimulation pulse generators, sense amplifiers and leads which can be configured or programmed to operate in single or dual chamber modes of operation, delivering pacing stimuli to the atrium and/or ventricle at fixed rates or rates that vary between an upper rate limit and a lower rate limit. More recently, single and dual chamber pacemakers have been developed that respond to physiologic sensors which, with greater or lesser degrees of specificity, sense the body's need to deliver more or less oxygenated blood to the cardiovascular system.
For example, rate responsive pacing systems have been developed and marketed which rely upon the patient's level of physical activity. Such pacemakers include the Medtronic Activitrax.RTM., Legend.TM. and Synergyst.TM. single chamber and dual chamber rate responsive pacemakers. The activity sensor of such pacemakers comprises a piezoelectric crystal bonded to the interior surface of the pacemaker pulse generator can and coupled through activity conditioning circuitry to digital controller circuitry. The output of the piezoelectric sensor varies as a function of the frequency or repetition rate of the patient's activity. The conditioned output signal is employed in the digital controller circuitry to select an appropriate pacing rate sufficient to increase or decrease the supply of oxygenated blood appropriate to the level of activity.
The activity sensors (which may be obtained from Vernitron Corporation) are uniformly shaped piezoelectric crystals sandwiched between two planar electrodes, one of which is bonded to the case and the other is connected to the input of the activity conditioning circuit. While the piezoelectric crystals ordered for any specific pulse generator model are relatively uniform in specifications relating to their size and electrical output, the manufacturing process of bonding the crystals to the pacemaker can, then adding and interconnecting the remaining components within insulated carriers fitted inside the can-halves, and laser welding the two halves of the can together imparts loads and stress upon the crystal affecting its response characteristics, much as a drumhead may be affected by tightening or loosening its hold down mechanism. Thus, it is necessary to first conduct tests of the electrical output of the piezoelectric crystal sensor after it is bonded to the can-half and to then test the sensor derived pacing rate response after the two can-halves are welded together to insure that the sensor output remains within specifications in the first instance and the desired range of rate response can be achieved in the second instance. Because of the relatively tight specifications and the manufacturing induced stresses, a certain fraction of the shield can-half assemblies and the finally assembled pulse generator fail to meet specifications and must be scrapped or reworked. Consequently, the cost of producing such devices is increased.
In addition, it would be desirable to place certain information, e.g., a serial number, model number, manufacturing series and/or date, into nonvolatile memory after the device is completely or virtually completely assembled to trace the completed device through its remaining steps of manufacture, sale and subsequent service or warranty tracking. Lastly, even device functions or modes of operation may be optimally changed after device manufacturing steps are completed.